Project Summary/Abstract Achilles tendinopathy has an incidence rate of 2.35 per 1000 in the general population and is most prevalent in middle-aged individuals (35-55 y/o), but occurs in men and women of all ages. The primary symptom is pain during daily activities such as walking and exercising such as running. Aside from the pain, Achilles tendinopathy has been shown to significantly decrease physical activity level, resulting in further negative effects on overall health and well-being. The treatment for Achilles tendinopathy with the highest level of evidence is eccentric exercise, providing mechanical loading of the muscle-tendon unit. In a recent systematic review, all studies reported significant improvements in patient-reported symptoms but at 12 weeks the means ranged from 69-80 (100 being fully recovered) indicating that even with the most effective treatment individuals continued to have symptoms. At this time, other more invasive interventions such as injection therapies (ex. platelet-rich plasma) and surgery are recommended for patients who fail exercise treatment despite a lack of understanding of what factors are related to continued problems. Just achieving a reduction in pain and symptoms with treatment also does not ensure resolution of the tendon's structural abnormalities. In fact, studies evaluating the recovery of tendon structure with exercise suggest that at least 24 weeks may be needed to observe a significant change. Other individual factors such as sex, degree of tendon structural damage and functional deficits are also proposed to influence both the time course and success rate of recovery. The long-term goal of our research is to advance understanding of tendon injuries and repair, enabling tailored treatments to be developed. This study begins to address this long-term goal by evaluating the time-course of recovery in terms of tendon structure (ultrasound imaging) and viscoelastic properties (elastography) along with symptoms (patient-reported outcomes) and muscle-tendon function (functional test-battery) in men and women with Achilles tendinopathy treated with an exercise program. Aim 1 is to evaluate if there are differences in change over time in symptoms, muscle-tendon function, tendon structure, and mechanical properties between males and females with Achilles tendinopathy receiving exercise treatment. Aim 2 is to investigate whether the presence and magnitude of tendon structural abnormality at baseline will affect the ability and time-course of recovery with exercise treatment for Achilles tendinopathy. Aim 3 is to explore if patients who continue to have symptoms at the 16-week evaluation will further improve in symptoms, muscle-tendon function, tendon structure and mechanical properties over the course of one year. This study will be the first to evaluate if there is a difference in recovery of tendon structure and mechanical properties between males and females with Achilles tendinopathy receiving exercise treatment. It will also be the first study to evaluate recovery of tendinopathy with exercise intervention using outcome measures for tendon structure and mechanical properties along with validated measures of muscle-tendon function and symptoms.